Magnetoresistive elements each having a MTJ (Magnetic Tunnel Junction) film formed by CoFeB/MgO/CoFeB (hereinafter, “/” will be used as “an upper layer/a lower layer” in relation to the substrate) have high magnetoresistance ratios (MR ratios), and therefore, have been under consideration for the use in the fields of MRAMs (Magnetic Random Access Memories), magnetic heads, and magnetic sensors. To achieve a high MR ratio with a MTJ film formed by CoFeB/MgO/CoFeB, the MTJ film needs to be subjected to a heat treatment at a temperature of about 500° C. The reason that a 500° C. heat treatment is performed on the MTJ film formed by CoFeB/MgO/CoFeB is that 500° C. is the crystallization temperature at which Co40Fe40B20 in an amorphous state is crystallized. For example, the transformation temperature at which CoFeB in an amorphous state turns into CoFeB in a crystallized state becomes higher as the B density becomes higher. When the B density is about 20%, the transformation temperature reaches 500° C.
However, the 500° C. heat treatment performed on the MTJ film formed by CoFeB/MgO/CoFeB induces diffusion of other elements forming the magnetoresistive element. Therefore, the 500° C. heat treatment cannot be performed on MTJ films formed by CoFeB/MgO/CoFeB used in MRAMs. For example, in a typical spin-valve film, a stacked film such as CoFeB/MgO/CoFeB/Ru/CoFe/PtMn is used. When this stacked film is subjected to a heat treatment at 400° C. or higher, mutual diffusion occurs among the elements contained in the synthetic antiferromagnetically-coupled film. The synthetic antiferromagnetic coupling is degraded, and the MR ratio becomes lower. Also, the Mn in the antiferromagnetic film made of PtMn diffuses into the MTJ formed by CoFeB/MgO/CoFeB, and the spin polarizability becomes lower, which leads to a reduction of the MR ratio. Further, in the synthetic antiferromagnetically-coupled film formed by CoFeB/Ru/CoFe, the synthetic antiferromagnetic coupling is broken due to the mutual diffusion among the respective elements, and the magnetization directions of the CoFeB layer and the CoFe layer become parallel to each other. Due to a magnetic field leaking from the CoFeB layer, a shift is caused in the magnetization characteristics of the CoFeB layer as the recording layer, resulting in the problem of write errors.
By conventional techniques, the upper limit of the temperature that can be applied to a MTJ is about 350° C., and therefore, it is difficult to achieve a high MR ratio. If a MTJ film formed by CoFeB/MgO/CoFeB can be completely crystallized at a temperature as low as 350° C., a magnetoresistive element having a high MR ratio can be obtained. In that case, the read output can be improved, and the spin polarizability can be made higher, which leads to a reduction of the write current. However, there have been neither reports nor specific methods suggested to achieve a high MR ratio by accelerating crystallization of the CoFeB layer through a low-temperature heat treatment performed on a MTJ film formed by CoFeB/MgO/CoFeB.